Tyres designed to rotate in one direction only or with an asymmetric tread pattern must be fitted the right way round
Standard car tyres have a symmetrical tread pattern that is the same across the whole width of the tyre. The tread pattern on an asymmetric car tyre changes across the tread with different parts adapted for particular functions.
The outside edge consists of large stiffer tread blocks which help with cornering. The inner tread blocks are smaller and designed to shift water and improve wet grip. The middle of the tread usually has a continuous rib to help straight line stability.
Asymmetric tyres must be fitted the right way round on the wheel to benefit from the tread block arrangement. The sidewalls will be clearly marked.
Directional tyres usually have a chevron or arrow pattern in the tread and are designed to work in only one direction of rotation.
Directional tyres disperse water that builds up in front of the tyre more effectively, reduce road noise, and improve directional stability.
They must rotate in the right direction and this will be clearly marked on the sidewall of the tyre.
If a directional tyre is fitted the wrong way round, the tyre won't be dangerous, but you won't gain any of the benefits of it's design.
Incorrect fitting of both asymmetric and directional tyres is a reason for MOT failure.
Some high performance cars are fitted with directional and asymmetric tyres. These must be fitted the right way round on the wheel and on the correct side of the vehicle.
(25 April 2012)